High-Voltage DC Breakthrough Could Boost Renewable Energy

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High-voltage DC transmission lines carry electricity from China's massive Three Gorges dam, the largest power plant in the world. Advocates of HVDC think it has an even greater role to play in bringing renewable energy to the grid.

Photograph by Zheng Jiayu, Xinhua Press/Corbis

Thomas Edison championed direct current, or DC, as a better mode for delivering electricity than alternating current, or AC. But the inventor of the light bulb lost the War of the Currents. Despite Edison's sometimes flamboyant efforts—at one point he electrocuted a Coney Island zoo elephant in an attempt to show the technology's hazards—AC is the primary way that electricity flows from power plants to homes and businesses everywhere. (Related Quiz: "What You Don't Know About Electricity")

But now, more than a century after Edison's misguided stunt, DC may be getting a measure of vindication.

An updated, high-voltage version of DC, called HVDC, is being touted as the transmission method of the future because of its ability to transmit current over very long distances with fewer losses than AC. And that trend may be accelerated by a new device called a hybrid HVDC breaker, which may make it possible to use DC on large power grids without the fear of catastrophic breakdown that stymied the technology in the past. (See related photos: "World's Worst Power Outages.")

Swiss-based power technology and automation giant ABB, which developed the breaker, says it may also prove critical to the 21st century's transition from fossil fuels to renewable energy sources, by tapping the full potential of massive wind farms and solar generating stations to provide electricity to distant cities.

So far, the device has been tested only in laboratories, but ABB's chief executive, Joe Hogan, touts the hybrid HVDC breaker as "a new chapter in the history of electrical engineering," and predicts that it will make possible the development of "the grid of the future"—that is, a massive, super-efficient network for distributing electricity that would interconnect not just nations but multiple continents. Outside experts aren't quite as grandiose, but they still see the breaker as an important breakthrough.

"I'm quite struck by the potential of this invention," says John Kassakian, an electrical engineering and computer science professor at the Massachusetts Institute of Technology. "If it works on a large scale and is economical to use, it could be a substantial asset."

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Going the Distance

The hybrid HVDC breaker may herald a new day for Edison's favored mode of electricity, in which current is transmitted in a constant flow in one direction, rather than in the back-and-forth of AC. In the early 1890s, DC lost the so-called War of the Currents mostly because of the issue of long-distance transmission.

In Edison's time, because of losses due to electrical resistance, there wasn't an economical technology that would enable DC systems to transmit power over long distances. Edison did not see this as a drawback because he envisioned electric power plants in every neighborhood.

But his rivals in the pioneering era of electricity, Nikola Tesla and George Westinghouse, instead touted AC, which could be sent long distances with fewer losses. AC's voltage, (think of it as analogous to the pressure in a water line), could be stepped up and down easily through the use of transformers. That meant high-voltage AC could be transmitted long distances until it entered neighborhoods, where it would be transformed to safer low-voltage electricity.

Thanks to AC, smoke-belching, coal-burning generating plants could be built miles away from the homes and office buildings they powered. It was the idea that won the day, and became the basis for the proliferation of electric power systems across the United States and around the world.

But advances in technology ultimately made it possible to transmit DC at higher voltages. The advantages of HVDC then became readily apparent. Compared to AC, HVDC is more efficient—a thousand-mile HVDC line carrying thousands of megawatts might lose 6 to 8 percent of its power, compared to 12 to 25 percent for a similar AC line. And HVDC would require fewer lines along a route. That made it better suited to places where electricity must be transmitted extraordinarily long distances from power plants to urban areas. It also is more efficient for underwater electricity transmission.

In recent years, companies such as ABB and Germany's Siemens have built a number of big HVDC transmission projects, like ABB's 940-kilometer (584-mile) line that went into service in 2004 to deliver power from China's massive Three Gorges hydroelectric plant to Guangdong province in the South. In the United States, Siemens for the first time ever installed a 500-kilovolt submarine cable, a 65-mile HVDC line, to take additional power from the Pennsylvania/New Jersey grid to power-hungry Long Island. (Related: "Can Hurricane Sandy Shed Light on Curbing Power Outages?") And the longest electric transmission line in the world, some 2,500 kilometers (1,553 miles), is under construction by ABB now in Brazil: The Rio-Madeira HVDC project will link two new hydropower plants in the Amazon with São Paulo, the nation's main economic hub. (Related Pictures: "A River People Await an Amazon Dam")

But these projects all involved point-to-point electricity delivery. Some engineers began to envision the potential of branching out HVDC into "supergrids." Far-flung arrays of wind farms and solar installations could be tied together in giant networks. Because of its stability and low losses, HVDC could balance out the natural fluctuations in renewable energy in a way that AC never could. That could dramatically reduce the need for the constant base-load power of large coal or nuclear power plants.

The Need for a Breaker

Until now, however, such renewable energy solutions have faced at least one daunting obstacle. It's much trickier to regulate a DC grid, where current flows continuously, than it is with AC. "When you have a large grid and you have a lightning strike at one location, you need to be able to disconnect that section quickly and isolate the problem, or else bad things can happen to the rest of the grid," such as a catastrophic blackout, explains ABB chief technology officer Prith Banerjee. "But if you can disconnect quickly, the rest of the grid can go on working while you fix the problem." That's where HVDC hybrid breakers—basically, nondescript racks of circuitry inside a power station—could come in. The breaker combines a series of mechanical and electronic circuit-breaking devices, which redirect a surge in current and then shut it off. ABB says the unit is capable of stopping a surge equivalent to the output of a one-gigawatt power plant, the sort that might provide power to 1 million U.S. homes or 2 million European homes, in significantly less time than the blink of an eye.

While ABB's new breaker still must be tested in actual power plants before it is deemed dependable enough for wide use, independent experts say it seems to represent an advance over previous efforts. (Siemens, an ABB competitor, reportedly also has been working to develop an advanced HVDC breaker.)

"I think this hybrid approach is a very good approach," says Narain Hingorani, a power-transmission researcher and consultant who is a fellow with the Institute of Electrical and Electronics Engineers. "There are other ways of doing the same thing, but they don't exist right now, and they may be more expensive."

Hingorani thinks the hybrid HVDC breakers could play an important role in building sprawling HVDC grids that could realize the potential of renewable energy sources. HVDC cables could be laid along the ocean floor to transmit electricity from floating wind farms that are dozens of mile offshore, far out of sight of coastal residents. HVDC lines equipped with hybrid breakers also would be much cheaper to bury than AC, because they require less insulation, Hingorani says.

For wind farms and solar installations in the Midwest and Rocky Mountain regions, HVDC cables could be run underground in environmentally sensitive areas, to avoid cluttering the landscape with transmission towers and overhead lines. "So far, we've been going after the low-hanging fruit, building them in places where it's easy to connect to the grid," he explains. "There are other places where you can get a lot of wind, but where it's going to take years to get permits for overhead lines—if you can get them at all—because the public is against it."

In other words, whether due to public preference to keep coal plants out of sight, or a desire to harness the force of remote offshore or mountain wind power, society is still seeking the least obtrusive way to deliver electricity long distances. That means that for the same reason Edison lost the War of the Currents at the end of the 19th century, his DC current may gain its opportunity (thanks to technological advances) to serve as the backbone of a cleaner 21st-century grid. (See related story: "The 21st Century Grid: Can we fix the infrastructure that powers our lives?")

Editor's Note: An earlier version of this story incorrectly said that advances in transformer technology made it possible to transmit DC long distances at high voltage. The story now reflects that other technological advances, not improvements in transformers, made that possible. The definitions of voltage and of AC also were amended for clarity.